vnc: avoid write only variables

vnc: avoid write only variables

Compiling with GCC 4.6.0 20100925 produced warnings:
/src/qemu/ui/vnc.c: In function 'vnc_client_cache_auth':
/src/qemu/ui/vnc.c:217:12: error: variable 'qdict' set but not used [-Werror=unused-but-set-variable]
/src/qemu/ui/vnc.c: In function 'vnc_display_open':
/src/qemu/ui/vnc.c:2526:9: error: variable 'acl' set but not used [-Werror=unused-but-set-variable]

Fix by making the variable declarations and their uses also conditional
to debug definition.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>